I've been looking at the threading dial on my AL320G and thinking it must be usable even though the "manual" that comes with the lathe doesn't tell you anything other than how to set up the change gears.
So first off some research into threading on the lathe by reading Workshop Practice Series #3: Screwcutting on the Lathe by Martin Cleeve.
Now this is pretty jolly complete and after reading things several times I felt I had enough of a handle on things to make the next steps.
So looking at the threading dial the gear is 20 teeth, the lead screw is a 3mm pitch and the dial has 4 "lines" or sectors on it.
So what does all this tell us ... well after some manual measurements the following ...
We get 60m travel of the saddle from a single rotation of the threading dial and the lead screw turns 20 times during that travel.
i.e. 20 teeth x 3mm pitch = 60mm travel.
So back to the book, specifically from pages 71 with specific attention to pages 81 and 82. So from reading this section it appears that the key to being able to use the threading dial is that the pitch of the thread needs to fit some rules with regard to:
 the thread pitch being a whole multiple of the leadscrew pitch
 the thread pitch being able to divide evenly into the amount of saddle travel represented by sectors on the dial
So armed with this hopefully correct info I built a chart to see what pitched threads I could use the dial for on the lathe and which I could not. And that is the kicker with metric threads, not all of them will work with the threading dial without some changing in the threading dials gearing.
The calculated table of threads. Blue lines show thread pitches not possible with the stock threading dial gearing.
Pic #2 & #3:
Now to do a proof. I picked a 1.5mm thread, setup the lathe gears, found some scrap and got it ready to do some light cuts.
So, set up the and ready to go I had the lathe running nice and slow (60rpm) and leadscrew turning and ready to engage the half-nut on a any whole sector line on the dial. Let it run a light scratch, disengaged the half-nut, pulled back the cross slide, brought the saddle back to start position, shutdown and checked with a gauge. Photo's not the best but you can see that the scratch is bang on the 1.5mm pitch gauge. So far so good. Now for the moment of truth, can we get repeated pickup?
Well repeated the same process and engaged the half-nut on one of the whole sector lines. Not the same as the last one, just to make sure there was some more variability in the mix. Re checked the slightly deeper cut, BINGO, bank on the same lines as last time.
Okay according to my understanding from the book learning I should be able to engage the half-nut at ANY point on the dial gauge as the 1.5mm is a whole multiple of the leadscrew pitch. So thats what I did, engaged the half-nut at a point between the sector lines on the threading dial.
And the result was bang on!
I ran out of time tonight to do some more variations on threads, also will need to dig out some more scrap round to trial on. But so far the math seems sounds and practical testing is lining up with the book learning.
Conclusion at this point is that while not all threads can be done using this method a good chunk of them can be so the threading dial is no longer looking like the useless appendage it seemed to be for metric threading.
I'll do some trials over the course of the week plus I want to look into what gearing would be needed on the thread dial to do the other threads and if that is even worth the hassle given the non-disengaged half-nut method will work just fine on those.